1. Field of the Invention
The present invention relates to methods for removing volatile organic material from an aqueous dispersion of a synthetic polymer. In particular, it relates to the removal of volatile organic contaminants (VOCs) from latex dispersions.
2. Description of Related Art
In the polymerization of organic monomers in an aqueous medium to form a dispersion of polymer in water, the resultant dispersion typically contains residual organic impurities which result from incomplete conversion of monomers, impurities in raw materials and undesirable by-products formed during the polymerization reaction. For example, in a styrene-butadiene aqueous emulsion polymer, these impurities can include: unreacted styrene and butadiene; ethyl benzene, an impurity in the styrene; 4-vinyl cyclohexene (VCH), an impurity in the butadiene; 4-phenyl cyclohexene (PCH), a by-product of the polymerization.
Conventionally, organic volatiles have been removed from an aqueous polymer dispersion made by emulsion or suspension polymerization by a process known as steam or inert gas stripping. In this process the dispersion is contacted with steam, air or an inert gas (e.g., nitrogen) at either reduced or elevated temperatures and pressures. This process can be carried out in a variety of different types of equipment, for example, columns, semibatch strippers, thin film evaporators and plate evaporators. However, because of the strong interaction between the organic volatiles and the polymer, to achieve a low level of volatiles in the dispersion requires high ratios of stripping medium/dispersion. To generate this stripping medium requires the use of large amounts of energy and results in high energy costs.
Another method which has been used for removing VOCs from natural and synthetic polymeric dispersions involves contacting the dispersion with an adsorbent material, such as carbon black, activated charcoal, silica gel, aluminum oxide and ferric oxide. A similar method is known for purifying various polymer resins wherein the polymer is first dissolved in a solvent and the resulting solution is contacted with alumino-silicates. Some prior art processes for decolorizing or deodorizing polymeric materials have suggested that stripping and adsorbent treatment could be used alternatively or even serially.
None of the prior art processes for purification of aqueous polymeric dispersions has been able to provide extremely low residual VOC levels under conditions that are both effective and economical.
The present invention overcomes the problems and disadvantages associated with current strategies and designs in purification of polymer dispersions.
The present invention relates to a process for removing VOCs from a polymer dispersion which process, in the preferred embodiment, comprises steam, air or inert gas-stripping the dispersion in the presence of a particulate adsorbent. This process can be employed to purify any polymeric dispersion, emulsion, suspension or solution and is particularly useful in the removal of VOCs from aqueous polymeric latices such as those based on styrene butadiene polymers or acrylic polymers. This process provides for the rapid and economical removal of VOCs. The process of this invention can result in achieving lower residual VOC levels than the prior art methods employing either steam stripping or particulate adsorbent treatment and can be employed to achieve similar VOC levels at lower energy costs than prior art methods. In one embodiment of the process of this invention, a slurry of adsorbent particles in the polymer dispersion is subjected to steam stripping, followed by separating the adsorbent particles from the purified latex dispersion. In another embodiment of this invention, the adsorbent particles are retained in the stripping vessel in the form of a fixed or fluidized bed or otherwise immobilized in said stripping vessel. In yet another embodiment, discrete quantities of the particulate adsorbent material are provided in latex permeable flow-through enclosures (e.g., in a manner analogous to teabags). These adsorbent-containing enclosures can be added to the stripping vessel separately or with the latex feed. Such an arrangement greatly facilitates the separation of spent adsorbents from the purified polymeric stream. If desired, both the stripping media and the adsorbent can be further processed to regenerate the reagents and recover valuable VOCs such as residual monomers.
Other objects, advantages and embodiments of the invention are set forth in part in the description which follows, and in part, will be obvious from this description, or may be learned from the practice of the invention.